Phase relations in the subsolidus area of the MgO–P2O5–Cr2O3 system were reinvestigated. The reactions were carried out by the method of multi-stage calcination of samples in the temperature range of 350–1150 °C. As a result of the conducted research, a new pyrophosphate with the formula Mg5Cr2 (P2O7)4 was discovered. This compound, crystallizing in the orthorhombic system, most likely exhibits the Fe 5 2+ Fe 2 3+ (P2O7)4 structure. The infrared spectrum of the new pyrophosphate was presented. Mg5Cr2 (P2O7)4 melts incongruently at 1330 °C depositing Mg3Cr4 (PO4)6. The developed division of the MgO–P2O5–Cr2O3 system into subsidiary subsystems differs significantly from the one known from the literature. First of all, unlike in the literature, it takes into account the existence of the compound just discovered, as well as the other two phosphates, i.e. Mg3Cr4 (PO4)6 and Cr2P4O13. The melting points of mixtures of phases coexisting in the equilibrium state were estimated by the pyrometric method. All phosphates (V) with mixed cations, i.e. Mg5Cr2 (P2O7)4, Mg3Cr4 (PO4)6 and MgCr2 (P2O7)2, were tested for their use for colouring lead-free glaze fired once at 1050 °C. It turned out that the investigated phosphates act as dyes and ceramic layers of good quality and interesting colours were obtained. The optimal concentration of the applying phosphates was close to 3 mass%.
